Qt uses standard C++, but
makes extensive use of the C pre-processor to enrich the language.
Qt can also be used in several other programming languages via language
bindings. It runs on all major platforms, and has extensive internationalization support.
Non-GUI features include SQL
database access, XML parsing, thread management, network
support and a unified cross-platform API for file
handling.

History

Haavard Nord and Eirik Chambe-Eng (the original developers of Qt
and the CEO and President, respectively, of Trolltech) began
development of "Qt" in 1991, three years before the company was
incorporated as Quasar Technologies, then changed the name to Troll
Tech, and then to Trolltech.

The toolkit was called Qt because the letter Q looked appealing in Haavard's Emacs font, and "t" was inspired by Xt, the X toolkit.[4]

Controversy erupted around 1998 when it became clear that KDE was going to become one of the
leading desktop environments for Linux. As KDE was based on Qt, many
people in the free software movement worried
that an essential piece of one of their major operating systems
would be proprietary.

The first two versions of Qt had only two flavours: Qt/X11 for
Unix and Qt/Windows for the Windows platform. The Windows platform
was only available under the proprietary license, which meant
free/open source applications written in Qt for X11 could not be
ported to Windows without purchasing the proprietary edition. In
the end of 2001, Trolltech released Qt 3.0, which added support for
the Mac OS X platform. The Mac OS X support was available only in
the proprietary license, until June 2003, where Trolltech released
Qt 3.2 with Mac OS X support available under the GPL.

In 2002 members of the KDE on Cygwin project began porting the
GPL licensed Qt/X11 code base to Windows.[5] This
was in response to Trolltech's refusal to license Qt/Windows under
the GPL on the grounds that Windows was not a free software/open
source platform.[6][7] The
project achieved reasonable success although it never reached
production quality.

This was resolved when Trolltech released Qt/Windows 4 under the
GPL in June 2005. Qt 4 now supports the same set of platforms
in the free software/open source editions as in the proprietary
edition, so it is now possible to create GPL-licensed free/open
source applications using Qt on all supported platforms.

Nokia acquired Trolltech ASA in 2008 and changed the name first
to Qt Software, then to Qt Development Frameworks. Since then it
has focused on Qt development to turn it into the main development
platform for its devices, including a port to the SymbianS60
platform. Also the source code was made available over Gitorious, a community
oriented git
source code repository, in order to gather an even broader
community that is not just using Qt, but also helping to improve
it.

License

At all times, Qt was available under a commercial license, which
allows the development of proprietary applications without
restrictions on licensing. In addition to that, Qt was at all times
licensed under an increasing number of increasingly free
licenses:

Until version 1.45, source code for Qt was released under the
FreeQt license — which was viewed as not compliant with the open
source principle by the Open Source Initiative and the
free software definition by Free Software Foundation,
because while the source was available it did not allow the
redistribution of modified versions.

With the release of version 2.0 of the toolkit, the license was
changed to the Q Public License (QPL), a free software
license but one regarded by the Free Software Foundation as
incompatible with the GPL. Compromises were sought between KDE and
Trolltech whereby Qt would not be able to fall under a more
restrictive license than the QPL, even if Trolltech were bought out
or went bankrupt. This led to the creation of the KDE Free Qt foundation,
which guarantees that Qt would fall under a BSD-style license should no free
software/open source version of Qt be released during 12
months.

Later, the GPL v2 and subsequently v3
with special exception[8] were
added as licensing options. The GPL exception allows the final
application to be licensed under various GPL-incompatible free software/open
source licenses such as the Artistic License.

As announced on January 14, 2009, Qt version 4.5 adds another
option, the LGPL,[9] which
should make Qt even more attractive for non-GPL open source
projects and for closed applications.[10]

All editions support a wide range of compilers, including the GCC C++ compiler and the Visual Studio suite.

Scribe A Unicode text renderer with a
public API for performing low-level text layout.

MainWindow A modern action-based main
window, toolbar, menu, and docking architecture.

Qt 4.1, released on December 19, 2005, introduced integrated SVG Tiny support, a PDF backend to Qt's printing
system, and a few other features.

Qt 4.2, released on October 4, 2006, introduced Windows Vista
support, introduced native CSS support for widget styling, as well
as the QGraphicsView framework for efficient rendering of thousands
of 2D objects onscreen, to replace Qt 3.x's QCanvas class.

Qt 4.4, released on May 6, 2008. Features included are improved
multimedia support using Phonon, enhanced XML support, a concurrency framework to ease the
development of multi-threaded applications,
an IPC framework with a focus
on shared memory, and WebKit
integration.

Qt 4.5, released on March 3, 2009. Major included features are
QtCreator, improved graphical engine,
improved integration with WebKit, OpenDocument Format write support and new
licensing options, as well as Mac OS X Cocoa framework support.

Qt 4.6, released on December 1, 2009. New APIs are Framework
Animation, Gestures, Multi-touch. Now supports (as Tier 1) Symbian
and (as Tier 2) Windows 7 and Mac OS X 10.6, support extended for
some UNIX systems. Also, great improvement in performances.

Bindings

Qt has a range of bindings for various languages that
implement some or all of its widget set.

Migration
tools

Design

The innovation of Qt when it was first released relied on a few
key concepts.

Use of native UI-rendering
APIs

Qt used to emulate the native look of its intended platforms,
which occasionally led to slight discrepancies where that emulation
was imperfect. Recent versions of Qt use the native APIs of the
different platforms to draw the Qt controls, and so do not suffer
from such issues.[26]

Meta object
compiler

Known as the moc, this is a tool that is run on the
sources of a Qt program. It interprets certain macros from the C++
code as annotations, and uses them to generate additional C++ code
with "Meta Information" about the classes used in the program. This
meta information is used by Qt to provide programming features not
available natively in C++: the signal/slot system, introspection and asynchronous
function calls.

Criticism

The use of an additional tool has been criticized for making Qt
programming different from pure C++ programming. In particular, the
choice of an implementation based on macros has been criticized for
its absence of type safety and pollution of the namespace.[2]Implementations
in native C++ exist in other libraries, but Trolltech viewed
macros as a necessary trade-off to provide introspection and the
dynamically generated slot and signal mechanism.[3]

QtScript ECMAScript
interpreter

Qt Script
for Applications is a cross-platform toolkit that allows
developers to make their Qt/C++ applications scriptable using an
interpreted scripting language: Qt Script (based on ECMAScript/JavaScript).

From Qt 4.3.0 onward, the scripting API,[27] which
is based on QSA,[28] is
integrated as a core part of Qt and is no longer a separate
library.